High density corrosive resistant gas to air heat exchanger

ABSTRACT

A gas to air heat exchanger includes corrosive resistant tubes made from one material, and high thermal conductivity air fins made from another material. This construction allows for meeting heat transfer requirements in a spatially constrained application, such as over the road trucks, where a compressed mixture of recirculated exhaust gas and incoming air are compressed and then cooled before being supplied to the engine intake. In one example, the heat exchanger includes tubes made from stainless steel brazed to relatively thin copper air fins in a low temperature brazing process, and the tubes are brazed on respective ends to heads of stainless steel via a high temperature brazing process. This core is then joined to an aluminum inlet tank and possible non-metallic outlet tank via a mechanical crimping process that positions a seal between the tanks and the respective heads.

TECHNICAL FIELD

The present disclosure relates generally to gas to air heat exchangers, and more specifically to cooling potentially corrosive gases, such as engine exhaust, in an envelope with relatively tight spatial constraints.

BACKGROUND

In recent times, when an engine included a turbocharger and exhaust gas recirculation, it might be only the incoming air that is compressed via the turbocharger before being combined with recirculated exhaust gas that is supplied to the engine. Such an engine, for example, is shown in co-owned U.S. Pat. No. 6,526,753. More recently, there have arisen reasons for adding the exhaust gas to the incoming air before passing the combined mixture through the turbocharger for compression. The compressed exhaust gas/air mixture often needs to be cooled before being supplied to the engine intake. Because the exhaust gases can contain corrosive constituents, such as sulfuric and/or nitric acid, the wetted surfaces of the cooler can, and often will, corrode over time. After a prolonged period, the fluid isolation between the cooling tubes and the air fins can be undermined, and in more extreme situations, the inlet or outlet tank can become corroded leading to holes allowing the hot exhaust gases to vent to atmosphere.

Some heat exchanger applications have additional problematic constraints. For instance, the spatial envelope available in an over the road truck can severely limit the space available for inclusion of a necessary gas to air heat exchanger. When relying on construction techniques according to the conventional wisdom these spatial constraints can become even more acute. Typically, a heat exchanger will include tubes, air fins and heads all constructed from a similar material that are joined together in a conventional well known brazing process. However, when corrosion resistance is a substantial issue, and that problem is combined with a severe spatial constraint, the conventional wisdom in some instances will suggest that the cooling demands of a given engine system in a specific application, such as an over the road truck, simply can not be met in the space available. Completely, redesigning the remaining portion of the engine to gain additional volume for a gas to air heat exchanger is too expensive an option for realistic consideration.

The present disclosure is directed to overcoming one or more of the problems set forth above.

SUMMARY OF THE DISCLOSURE

In one aspect, a gas to air heat exchanger comprises a core with a plurality of tubes that are fluidly isolated from, but in heat transfer contact with, a plurality of air fins. The tubes comprise a tube material, and the air fins are comprised of an air fin material. The tube material has a high corrosive resistance relative to the air fin material. The air fin material has a high thermal conductivity relative to the tube material.

In another aspect, an engine system includes a gas to air heat exchanger fluidly positioned in between a compressor outlet and an engine intake. An exhaust gas recirculation system is fluidly connected between an engine exhaust and a compressor inlet. The heat exchanger includes an inlet tank with a first minimum wetted wall thickness, a plurality of tubes with a second minimum wetted wall thickness, and a plurality of air fins with a third minimum wetted wall thickness. The first minimum wetted wall thickness is greater than the second minimum wetted wall thickness, which is greater than the third minimum wetted wall thickness.

In still another aspect, a method of making a gas to air heat exchanger includes a step of assembling a core out of at least two different materials in a two step braising process at high and low temperatures, respectively. A tank is mechanically attached to the core with a seal positioned between the two.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an engine system according to the present disclosure;

FIG. 2 is a schematic illustration of a gas to air heat exchanger according to the present disclosure;

FIG. 3 is a sectioned view looking into one of the tubes for the heat exchanger of FIG. 2; and

FIG. 4 is a partial sectioned corner view of a mechanical attachment between a head and tank portion of the heat exchanger of FIG. 2.

DETAILED DESCRIPTION

Referring now to FIG. 1, an engine system 10 includes a plurality of combustion cylinders 12 and at least one turbocharger 14. In the illustrated example, two turbochargers 14 include a pair of compressors 18 in series as well as a pair of turbines 20 in series, which are fluidly connected to exhaust manifold 16 in a conventional manner. Engine system 10 includes a gas to air heat exchanger 28 fluidly connected between a compressor outlet 22 and an engine intake 24 via a hot gas passage 26 and a cooled gas passage 30, respectively. Engine system 10 also includes an exhaust gas recirculation system 34 fluidly connected between an engine exhaust 36 and a compressor inlet 21. In particular, the exhaust gas recirculation system 34 includes an exhaust gas recirculation passage 39 fluidly connected to supply passage 31 via an EGR control valve control valve 40. Ambient air is drawn into supply passage 31 past an air filter 32 and through a valve 33 so that, along with EGR control valve 40, the relative amounts of exhaust gas and fresh air supplied to the engine can be controlled via an electronic control module (not shown) a conventional manner. The engine also includes one or more exhaust aftertreatment devices 35 positioned in exhaust passage 36, which may include a particle trap, an oxidation catalyst and the like exhaust passage 36 eventually terminates in a tail pipe 38.

Referring now in addition to FIG. 2, gas to air heat exchanger 28 includes a core 60 and an inlet tank 61 with an inlet 62 fluidly connected to hot gas passage 26, and an outlet tank 63 with an outlet 64 fluidly connected to cooled gas passage 30. Hot gases entering inlet 62 enter an inlet manifold area 73 and travel through a plurality of tubes 67 into outlet manifold area 75. The hot gases traveling through tubes 67 exchange heat with air traveling in a direction in and out of the page via air fins 66 in a conventional manner.

In order to meet tight spatial constraints while having superior heat transfer capability in the face of potentially corrosive gases, gas to air heat exchanger 28 includes a number of unique features. By putting an appropriate amount of the appropriate material in the right locations, gas to air heat exchanger 28 can provide adequate heat exchange while avoiding many of the problems associated with corrosive gases, and do so in a tight spatial envelope. From one perspective, this is accomplished by making the minimum wetted wall thickness of tanks 61 and 63 thicker than the minimum wetted wall thickness of tubes 67, which have a greater thickness than the minimum wetted wall thickness of air fins 66. Using this strategy, and realizing that the air fins need not be substantially corrosive resistant, they can be made of a relatively thin highly thermally conductive material, such as thin sheeting made predominantly of copper. Although not preferred, air fin material could also be cupro brazed copper, and less preferably a suitable stainless steel alloy, such as 409 stainless steel. Those skilled in the art will appreciate that the air fin material can include any of a variety of a materials exhibiting thermal conductive properties typical of the materials just identified. In any instance, the air fin material should be more thermally conductive than a tube material for tubes 67.

Like air fins 66, tubes 67 must have substantial thermal conductivity, but resistance to corrosion is also an important consideration. Those skilled in the art will recognize that the more thermally conductive a material is, generally the lower its ability to resist corrosion, and vice versa. With this in mind, tubes 67 might be made of stainless steel, with that being chosen in order of preference from 409 stainless steel, 304 and possibly even 316 stainless steel. Apart from stainless steel, tubes 67 might also be constructed from a suitable corrosive resistant material such as titanium, nickel plated aluminum, or possibly even nickel plated steel. Given these examples, those with ordinary skill in the art will recognize a family of materials that could be used for tubes 67 that have significant corrosive resistance, yet retain sufficient thermal conductivity for use in a heat exchanger application. As shown in FIG. 3, tubes 67 may or may not include internally brazed turbulators 78, which if included, would preferably be made of a material similar to that of its surrounding tube 67. The tube material should be more corrosive resistant than the air fin material.

As in a conventional heat exchanger, the gas to be cooled is isolated from air fins 66 by attaching heads 69 and 70 at opposite ends of tubes 67. Like turbulators 78, heads 69 and 70 are preferably made from a material similar to that of tubes 67, to ease the attachment between the two. Thus, in one specific example, the heads 69 and 70, as well as tubes 67 and turbulators 78, if any, would all be made from a common stainless steel material and then brazed to one another with a high temperature brazing material 71 in a conventional manner. Some suitable high temperature brazing alloys include 613 nickel based alloys, nickel plating alloys, and possibly even Bnix alloys. After brazing together the tubes 67, heads 69, 70 and any turbulator 78, the air fins are fitted between tubes 66 and attached to the tubes in a relatively low temperature brazing process that facilitates good heat transfer between the tubes 67 and air fins 66. Some suitable low temperature alloys might include OKC 600, nickel plating alloys, and copper based alloys. Those skilled in the art will appreciate that based upon these example brazing alloys, a number of different alternatives would be available without departing from the scope of the disclosure.

Inlet tank 61 must take into account other considerations, including but not limited to, corrosive resistance and cost considerations as well as high temperatures. With these considerations in mind, tank 61 could be constructed from aluminum, such as a cast aluminum alloy with relatively thick walls that can tolerate expected corrosive concentrations and durations without allowing corrosive holes to develop. Outlet tank 63 could be made of a like material, or further weight and cost savings might be achieved by employing some other material, such as a non-metallic composite since the gases arriving at outlet area 75 are much lower in temperature. Referring to FIG. 4, in order to mate the inlet and outlet tanks 61, 63 to the respective heads, 69 and 70, a mechanical attachment is preferably used by crimping the respective heads around an exposed flange on the respective tanks 61 and 63. Another mechanical attachment might include conventional fasteners, such as bolts or screws. In order to prevent gas from escaping, a suitable o-ring seal is positioned between the respective tank 61, 63 and its counterpart head 69, 70. Thus, heat exchanger 28 is preferably constructed by first employing a high temperature brazing process to assembly the heads 69, 70 to the tubes 67 and the interior turbulators 78, if any, in a high temperature brazing process using a suitable brazing alloy. Next, the highly thermally conductive air fins are attached to the tubes, which are typically made from a different and relatively thin material from that of the tubes 67 via a low temperature brazing process. The heat exchanger is then completed by mechanically attaching, such as via a crimping process, the external tanks 61 and 63 with an o-ring seal positioned there between.

INDUSTRIAL APPLICABILITY

The gas to air heat exchanger 28 according to the present disclosure finds potential application where corrosive gases need to be cooled with, but isolated from, air, and this cooling must be done in a relatively tight spatial constraint. For instance, in some work machines, such as over the road trucks, engines have evolved to include turbocharging and exhaust gas recirculation upstream from the compressor. When this occurs, the mixture of incoming air and exhaust gases must often need to be cooled prior to entry into the engine so that the engine can better function to achieve good efficiency and low emissions. Prior to such an engine evolution, the same over the road truck might have a simple air to air after cooler that did not need substantial corrosive resistance since there may not have been exhaust gas recirculation, but if there was, the exhaust gases were added to the intake downstream from the air cooler. But the spatial envelope available for gas cooling remained about the same, leading to the need for a high density corrosive resistant gas to air heat exchanger of a type described in this disclosure.

The gas to air heat exchanger seeks to address the needs of specific portions of the heat exchanger with materials having specific properties and in specific quantities (wall thicknesses) necessary to perform there needed function. For instance, the air fins need not necessarily be corrosive resistant but should be made from a highly thermally conductive material, such as one predominantly made of copper so that good heat transfer can occur from the air fins 66 to air passing through the heat exchanger 28. The tubes 67, on the other hand also need substantial heat exchanging capabilities, but this must be tempered with the need for corrosive resistance. Although the air fins 66 can be made extremely thin, the tubes 66 generally have a wetted wall thickness thicker than that of the air fins 67. The respective heads 69 and 70 attached to opposite ends of the tubes 67 should have a thickness on the order of that of the tubes and are preferably made of the same materials, for ease in attaching the two during a high temperature brazing process. The core 60 can be completed with a low temperature brazing process when attaching the relatively thin predominantly copper air fins 66 to the outer surfaces of the tubes 67. Although the tanks 61 and 63 could also be made from stainless steel, substantial costs savings can be achieved by making them from a less expensive material, such as cast aluminum, and possibly a composite for the cooler outlet tank 63. However, because aluminum has less corrosive resistance than stainless steel, the walls would generally have to be thicker than that of the tubes 67 and head 69 and 70 so that the inevitable corrosion to the wetted inner surface of the tanks could be tolerated over the expected life of the heat exchanger 28 without holes developing. Any problems associated with attaching aluminum and/or composite tanks to the stainless steel heads of core 60 are remedied via a mechanical attachment process, such as by crimping an extension of the heads about a flange on the respective tank 61 and 63. Before doing so, a suitable o-ring seal is positioned between the tank and head to inhibit leakage of corrosive gases from the heat exchange 28. Thus, the present disclosure brings a unique combination of heat exchanger features together, and assembles them in a unique way to arrive at a cost effective heat exchanger that can tolerate corrosive gases, and cool the same in a relatively small spatial volume.

It should be understood that the above description is intended for illustrative purposes only, and is not intended to limit the scope of the present disclosure in any way. Thus, those skilled in the art will appreciate that other aspects, objects, and advantages of the disclosure can be obtained from a study of the drawings, the disclosure and the appended claims. 

1. A gas to air heat exchanger comprising: a core with a plurality of tubes fluidly isolated from, but being in heat transfer contact with, a plurality of air fins; the tubes comprising a tube material, and the air fins comprising an air fin material; the tube material having high corrosive resistance relative to the air fin material; and the air fin material having high thermal conductivity relative to the tube material.
 2. The heat exchanger of claim 1 including first and second heads brazed to opposite ends of the tubes, respectively; first and second tanks mechanically attached to the first and second heads, respectively; and first and second seals operably positioned between the first head and the first tank, and between the second head and the second tank, respectively.
 3. The heat exchanger of claim 1 wherein the air fin material is predominantly copper; the tube material is predominantly stainless steel; and at least one of the first and second tanks comprise a tank material that is predominantly aluminum.
 4. The heat exchanger of claim 2 wherein at least one of the first tank and the second tank has a minimum wetted wall thickness that is greater than a minimum wetted wall thickness of the tubes, which is greater than a minimum wetted wall thickness of the air fins.
 5. The heat exchanger of claim 2 wherein the heads are attached to the tubes with a high temperature brazing material; and the air fins are attached to the tubes with a low temperature brazing material.
 6. The heat exchanger of claim 2 including a turbulator positioned in at least one of the tubes.
 7. The heat exchanger of claim 4 wherein the tube material is predominantly at least one of stainless steel, titanium, NI-plated aluminum and Ni-plated steel; the air fin material is predominantly at least one of copper, cuprobraze copper, and stainless steel; the heads are attached to the tubes with a high temperature brazing material that is predominantly at least one of Ni-based, Ni-plating and a Bnix alloy; the air fins are attached to the tubes with a low temperature brazing material that is predominantly at least one of OKC600, Ni-plating and Copper based.
 8. The heat exchanger of claim 2 wherein the air fin material is predominantly copper; the tube material is predominantly stainless steel; at least one of the first and second tanks comprise a tank material that is predominantly aluminum; at least one of the first tank and the second tank has a minimum wetted wall thickness that is greater than a minimum wetted wall thickness of the tubes, which is greater than a minimum wetted wall thickness of the air fins; the heads are attached to the tubes with a high temperature brazing material; and the air fins are attached to the tubes with a low temperature brazing material.
 9. An engine system comprising: a gas to air heat exchanger fluidly positioned between a compressor outlet and an engine intake; an exhaust gas recirculation system fluidly connected between an engine exhaust and a compressor inlet; the heat exchanger including an inlet tank with a first minimum wetted wall thickness, a plurality of tubes with a second minimum wetted wall thickness, and a plurality of air fins with a third minimum wetted wall thickness; and the first minimum wetted wall thickness is greater than the second minimum wetted wall thickness, which is greater than the third minimum wetted wall thickness.
 10. The engine system of claim 9 wherein the gas to air heat exchanger includes first and second heads brazed to opposite ends of the tubes, respectively; the inlet tank and an outlet tank being mechanically attached to the first and second heads, respectively; and first and second seals operably positioned between the first head and the inlet tank, and between the second head and the outlet tank, respectively.
 11. The engine system of claim 10 wherein the tubes comprise a tube material, the air fins comprise an air fin material, and the inlet tank comprises a tank material; and the tube material is more corrosive resistant than the tank material, which is more corrosive resistant than the air fin material.
 12. The engine system of claim 11 wherein the air fin material has higher thermal conductivity than the tank material, which has higher thermal conductivity than the tube material.
 13. The engine system of claim 12 wherein the first and second heads are brazed to opposite ends of the tubes with a high temperature brazing material; and the air fins are attached to the tubes with a low temperature brazing material.
 14. The engine system of claim 13 wherein at least one of the tubes includes a turbulator therein.
 15. The engine system of claim 14 wherein the air fin material is predominantly copper; the tube material is predominantly stainless steel; and the tank material is predominantly aluminum.
 16. A method of making a gas to air heat exchanger, comprising the steps of: assembling a core out of at least two different materials in a two step brazing process at high and low temperatures, respectively; and mechanically attaching a tank to the core with a seal positioned therebetween.
 17. The method of claim 16 wherein the assembling step includes the steps of: brazing high thermal conductivity air fins to low thermal conductivity tubes at the low temperature; and brazing a head to the tubes at the high temperature.
 18. The method of claim 17 wherein the mechanically attaching step includes a crimping process.
 19. The method of claim 17 wherein the high temperature brazing step includes brazing a turbulator inside at least one tube. 